The primary goal of the proposed study is to determine the course and termination of the axons forming efferent projections of the medial terminal nucleus (MTN) of the accessory optic system in the rabbit. To accomplish this, each of a series of rabbits will be injected unilaterally into the MTN with 3H-proline plus 3H-leucine. After survival periods that allow optimal amounts of anterograde neuronal transport of the tritiated label, brain series will be processed by the autoradiographic nerve tracing technique (ARNT), and by studying silver grain distribution, the axons arising from the MTN will be traced to their sites of termination in thalamic nuclei, brain stem nuclei and vestibulocerebellar subdivisions. Data provided will settle a discrepancy as regards the site of termination for the MTN-vestibulocerebellar projection in vertebrates. Specifically, this study will reveal the subdivisions of vestibulocerebellum (paraflocculus, uvula, flocculus and/or modulus) to receive input from the rabbit's MTN. The proposed research will further show the presence of a potential bisynaptic pathways from retina to oculomotor nucleus and interstitial nucleus of Cajal which in the rabbit might function in the reflex control of vertical eye and neck movements. Moreover, the proposed investigation will permit identification of other MTN projections as reported in anatomical studies on pigeons (Brecha et al., '80) or as postulated to provide input to pretectal nuclei and to dorsal and terminal accessory optic nuclei in electrophysiological experiments on rabbits (Maekawa and Simpson, '72, '73). A second goal of the proposed study will be to identify possible projections to specific vestibulocerebellar subdivisions from (1) cells located in ventral and dorsal divisions of the MTN and (2) small and large cell types present in the MTN. In order to achieve this, each of a series of rabbit will be injected with horseradish peroxidase (HRP) into a vestibulocerebellar subdivision shown in the ARNT experiments to receive input from the MTN. After allowing optimal time for the retrograde neuronal transport of HRP, the areal distribution within and size range of HRP labeled cells of the MTN will be determined.